The need to test and troubleshoot digital electronic circuits and equipment has precipitated a need for diagnostic apparatus which is capable of quickly locating faults or malfunctions, and which apparatus is also easy to use by service men or technicians. Currently available instruments such as oscilloscopes, logic probes, and logic state analyzers are presently used in diagnostic applications; however, the information provided by these instruments is limited or incomplete and subject to interpretation.
U.S. Pat. No. 3,573,751 describes a system for isolating faulty circuit modules in electronic equipment by the use of binary signature waveforms. A test sequence is provided whereby each module is energized to generate a signature waveform which is analyzed to determine its own particular class identifier. This analyzed class identifier is then compared with a stored class identifier of the correct signature waveform and as long as the signature class identifier signals match, the test sequence is advanced to the next module. Upon a mismatch of class identifier signals, the test sequence stops and a display indicator indicates the faulty module. Such a system is limited to locating a faulty module in a serial string of modules which may be addressed and tested in a sequence. In addition, the foregoing system employs a cumbersome system of signature analysis wherein binary waveforms are partitioned into a selected number of classes by summing, modulo N, binary numbers during a given interval to provide a quantitative value representing the class identification number. However, this summation approach to partitioning provides a one in N chance of a failed module going undetected because such a failed module will generate any signature class identifier with equal probability. Thus, for fast and accurate troubleshooting purposes, this system does not have sufficient reliability. Additionally, the foregoing system does not lend itself to isolating faults at the component level because the system does not provide for testing branched circuit paths.
A more accurate and less cumbersome method of signature analysis is described in U.S. Pat. No. 3,976,864. Here, serial bit streams are converted into hexadecimal signatures by means of a feedback shift register. The method employed by this patent in locating faulty components or modules is to apply a reference digital signal to a digital electronic circuit, touch a probe tip to a predetermined test point in the circuit, and observe the signature displayed by the test apparatus. This signature is mentally compared to a signature provided on the schematic of the circuit under test. However, even for a skilled technician or one having intimate knowledge of the circuit under test, such an approach is tedious, time consuming, and prone to error, particularly where troubleshooting a complex circuit.